3" Toilet Drain

I have a toilet drain on the 2nd floor directly above a 2x6 wall on the 1st floor. Can I run the 3" drain from the toilet flange straight down to the basement without any elbows at the top and tie into the horizontal drain line? I have a 2" vent tied into the 3" pipe about 18" below the toilet flange and vented up throught the roof. Someone said that they think this design might suck the water out of the toilet trap. Is this true?

It doesn't matter whether it drops straight down or has a few bends.
Hopefully the bowl "does" siphon when you flush. The fill valve refills the bowl after a flush anyway. I would have no problem with a toilet drain run straight down. Wait..........I've done hundreds that way.